The field of magnetic shielding is not well understood and is often referred to as a "black art." It is generally believed that shields of high conductivity materials such as copper or aluminum will reduce disturbing alternating fields generally referred to as "EMT" which stands for electromagnetic interference. Such shields are, indeed, quite effective with respect to higher frequency field components. Their effect, however, is limited to the depth of penetration of eddycurrents which is a function of conductivity and also of frequency. At low frequencies, such as 60 Hz, the thickness requirements would be such as to make shields of conductive materials completely impractical. For low frequencies, shields of high permeability material have been used with varying degrees of success. When such shields are of high permeability material or of large thickness and when the shield has no opening so as to completely surround an object to be shielded or when the direction of the shield is such that it does not penetrate any opening in the shield, the shield can be highly effective. However, in many cases, as in shielding cathode ray tubes, photo-multiplier tubes and the like, for example, an opening is required for visual or other access to the object and the orientation of the ambient field relative to the desired orientation of the object is either not stable or is not controllable.
Accordingly, it has not been possible to insure satisfactory results. The practice has been to use very high permeability materials, to limit the sizes of openings as much as possible and to hope for the best. If low frequency fields are still a problem, attempts may be made to locate the source of the disturbing field and to try erection of suitable shields close to the source. For example, if the source is a large electric motor, a large panel of a relatively high permeability material may be disposed between the motor and the object to be protected. In any event, it is frequently necessary to go to a great deal of trouble and expense to solve the problem if it can be solved at all.